This invention relates generally to current limiters and, more particularly, to limiters for limiting beam current in a high energy particle accelerator such as a cyclotron or linear accelerator.
High energy particle accelerators, such as cyclotrons and linear accelerators, are widely used in research laboratories and health care facilities. Because of the high energies involved and the consequent possibility of developing potentially harmful radiation, human safety requires that precautions be taken against conditions that could accidentally generate excessive or harmful radiation.
One potentially harmful condition results when the number of charged particles being accelerated (i.e., the beam current) unintentionally exceeds a pre-established upper limit. Through operator error or equipment malfunction, the actual beam current can exceed that which is desired. Such a condition is especially possible when a particular application requires that the accelerator be operated at less than its maximum capacity. In a medical treatment setting, such an overcurrent condition could result in an excessive dose of radiation being delivered to a patient. In a scientific research setting, such a condition could expose the research facility personnel to potentially harmful radiation.
In the past, excess radiation conditions have been avoided by placing radiation detectors at various locations in the accelerator facility. The detectors themselves were selected on the basis of the type, direction and field strength of the radiation anticipated. A computer-based system received information from the sensors and automatically shut down the accelerator in the event excessive radiation was sensed by one or more of the detectors. Although effective in avoiding prolonged exposure to excessive radiation, the system nevertheless had some deficiencies. Frequently, as much as one second was required for the system to detect excessive radiation and shut down the accelerator. A significant radiation overdose could be received by the operating personnel during that time. Furthermore, the effectiveness of the system depended on the proper selection and positioning of the radiation detectors. Thus, it was possible that some excess beam current events were never detected because they did not cause excessive radiation at the locations of the radiation detectors. Such events could, however, cause undetected excessive radiation where the radiation detectors were not located. Any personnel in such locations could unknowingly be exposed to excessive radiation.
In view of the foregoing, a need exists for a particle accelerator beam current limiter that directly senses the beam current with accuracy and reliability and that automatically terminates excessive beam current conditions before significant excessive radiation is developed.